Components having an array of a large number of minute through holes are used as microscopic elements for MEMS or electronic devices. As such components there are generally used silicon wafers whose expansion and contraction due to temperature change is small (CTE=around 35×10−7/° C.) and which are thus resistant to breakage. Silicon wafers, which have a low coefficient of thermal expansion (CTE), are also characterized by undergoing little change in properties in response to temperature change. However, production of monocrystalline silicon, which is a main material of silicon wafers, requires very high cost, so that silicon wafers are also very expensive. Furthermore, laser processing employing ablation, which is a practically used technique for forming holes in silicon wafers, necessarily involves applying a plurality of laser pulses to form one hole and has difficulty in achieving high-speed processing. That is, such laser processing employing ablation requires a long tact time and hence a high processing cost.
A technique is known that uses a combination of ultraviolet laser pulse irradiation and wet etching and that theoretically enables high-speed hole forming by which 1000 or more holes can be formed per second (Patent Literature 1). In this processing method, pulsed laser beams having a wavelength of 535 nm or less are focused by a specific lens, and a sheet of glass, in which holes are to be formed, is irradiated with the focused laser beams to form modified portions in the glass. The glass having the modified portions formed therein is immersed in hydrofluoric acid to form through holes or blind holes in the modified portions; this hole formation takes advantage of the fact that the modified portions are etched at a higher rate than the rest of the glass.
According to Patent Literature 1, collective and simultaneous formation of cylindrical or frustoconical through holes is achieved in a titanium-containing silicate glass as described in Example 1, 12, or the like. However, the composition of the glass disclosed in the examples includes such a large amount of alkali metal in the form of oxide that the glass has a higher coefficient of thermal expansion than silicon wafers and is unsuitable for use in MEMS or electronic devices. In addition, the glass contains a high concentration of titanium which is known as a component that can promote devitrification as a core former. The glass is thus prone to devitrification, which is disadvantageous for continuous production. Furthermore, an alkali component contained in the composition can contaminate device production lines.